The National Space Transportation System external tank is a sacrificial structure used for approximately eight and one-half minutes during each space launch. This external tank provides liquid oxygen and liquid hydrogen to the orbiter's main engines. The external tank is designed to carry sufficient propellant to place the external tank (attached to the orbiter) into low Earth orbit. Currently, the external tanks are jettisoned just prior to low Earth orbit insertion, and subsequently tumble and break up in the atmosphere before falling over open seas. The external tank consists of two pressure vessels, an oxygen tank and a hydrogen tank, joined by a cylindrical intertank structure.
In U.S. Pat. No. 4,807,833 (Pori), the configuring of a space vehicle fuel cell in order to provide modular structural building components for a space station is disclosed. The configuring of the fuel cell includes the modifying of existing intertank structure to provide two concentric fuel storage tanks.
Other patents of general interest include: U.S. Pat. No. 3,866,863 (Von Pragenau) which discloses a space vehicle using an external fuel cell; and U.S. Pat. No. 4,132,373 (Lang) which discloses a manned space flight body comprising a cylindrical shell.
It should also be appreciated that a nuclear fusion reaction applicable to electrical power generation includes the helium isotope of mass number three combining with deuterium (D) to yield a proton and a helium nucleus of mass four. The recoil energy from the charged particles can generate electrical power by direct conversion through an interaction with a magnetic field. In addition, the absorption of the recoil energy does not require the nuclear transmutations associated with fusion reactions that produce neutrons. The .sup.3 He reaction has been observed and characterized within present controlled fusion research, and the advantages associated with the charged particles appear to offset the higher input energies associated with containment and initiation.
Projections for fusion technology advances indicate that the D--.sup.3 He reaction could become a viable power system alternative coincident with the establishment of a permanent lunar base so that lunar sources would be available to supply Earth requirements. In particular, although the quantities of .sup.3 He available on Earth could not support a large scale generation of electrical power with this reaction, data from lunar samples indicate the presence of .sup.3 He in recoverable quantities sufficient to supply fusion powered generation of electricity. In addition, the gaseous by-product from the .sup.3 He extraction provide hydrogen and oxygen in a quantity to provide a transport propellant.
The transport of .sup.3 He encounters practical difficulties of low specific mass combined with the lowest liquification temperature (3.20K, 5.76 R) of any isotope. At standard atmospheric pressure, .sup.3 He remains a liquid at absolute zero.
It appears that the delivery of .sup.3 He to Earth at rates above 1,000 kg/month (2205 lb/month) would have a significant economic benefit. Preliminary comparisons of fuel costs relative to other sources show an economic advantage if .sup.3 He can be delivered to the Earth for a billion dollars per metric ton. Deliveries of about five metric tons per month would probably provide two times the electrical needs of the United States.